Web materials, generally planar materials having a thickness much smaller than the dimensions of the plane of the material are well known. Examples of web materials include metal foils, celluloid films, magnetic tapes, and paper products including hard grades of paper as well as tissue papers.
Handling web materials, and particularly handling lightweight and fragile web materials, without damaging the materials is facilitated by controlling the speed of the web handling machinery according to the tension of the web material. The machinery speed is adjusted to maintain the web tension at a value below the tension at which the web will break or be damaged. These control methods require the measurement of the web tension or of a value analogous to the web tension as a source of feedback for the machine controls.
Previously, tension has been measured with the use of an instrumented idler roller that is wrapped by the web material. These rollers can be problematic in that the roller has a mass therefore an inertial impulse force is necessary to start the roller moving. Once moving, the roller has inertia that must be overcome to slow or stop the roller as the web slows or stops. The impulse force and roller inertial forces can be sufficient to damage or break the web. Therefore, a method of measuring web tension without contacting the web is desired.
Previous non-contact methods detect local changes in the pressure of an air column that is coupled to the boundary air between the web material and a curved surface. These methods can be adversely affected by dust in the measurement area and may not be effective at very low tension levels associated with the handling of lightweight paper webs such as paper towels, and bath tissues, since the local changes in the boundary air layer associated with the changes in the low tension levels of such webs are small.